U.S. Pat. No. 4,318,699 discloses a sensor that responds to the operation of a marine transportation system to sense on-plane and off-plane conditions of a boat to operate a trim control to automatically position a trimmable drive for a desired boating operation. The preferred embodiment senses engine speed while an alternative embodiment senses fluid pressure opposing boat movement. The drive is moved to an auto-out position at high speeds and to a trimmed-in position at lower speeds.
U.S. Pat. No. 4,490,120 discloses a hydraulic system for trimming and tilting an outboard propulsion unit, which includes both trim piston-cylinder units and a trim-tilt piston-cylinder unit. The flow of hydraulic fluid from the reversible pump is controlled by a spool valve. A pressure relief valve is mounted in the spool to maintain pressure on one side of the spool when the pump is turned off to rapidly close the return valve and prevent further movement of the piston-cylinder units.
U.S. Pat. No. 4,776,818 discloses an electrical control system for trimming a pair of stern motors or drives mounted side-by-side on a boat. The two drives are both jointly and independently movable through a plurality of trim positions. The system includes two trim cylinders, each coupled to one associated drive, to move its associated drive to different trim positions both jointly as well as independently of each other. An operator controlled mechanism energizes and de-energizes the two trim cylinders simultaneously to jointly vary the trim position of the two drives. Two lines, each coupled at its first end to one associated drive, independently detect both the angular trim position of its associated drive with respect to the other drive as well as detect the trim position of the two drives jointly. Automatic control means coupled to the second end of each of the two lines is responsive to the two lines, when the two drives are not in the desired equal trim position with respect to each other, and controls switches to inactivate one of the trim cylinders and thereby move the other of the trim cylinders with respect to the inactivated one trim cylinder until the desired equal trim position is achieved between the two drives.
U.S. Pat. No. 6,007,391 discloses an automatically adjustable trim system for a marine propulsion system that provides automatic trimming of the propeller in response to increased loads on the propeller. A propulsion unit is attached to a boat transom through a tilt mechanism including a transom bracket and a swivel bracket. In a first embodiment, the transom bracket is clamped to a flexible transom which flexes in response to forces exerted on the transom during acceleration. In a second embodiment, the transom bracket is clamped to a transom bracket mounting platform that is generally parallel to and pivotally attached to the transom. A trim angle biasing mechanism is mounted between the transom and the transom bracket mounting platform for automatically adjusting the trim angle. A third embodiment includes a trim angle biasing mechanism incorporated into the transom bracket or swivel bracket. A fourth embodiment includes a spring-loaded pawl assembly between the swivel bracket and transom bracket.
U.S. Pat. No. 7,347,753 discloses a hydraulic system for a sterndrive marine propulsion device that directs the flow of hydraulic fluid through the body and peripheral components of a gimbal ring in order to reduce the number and length of flexible hydraulic conduits necessary to conduct pressurized hydraulic fluid from a pump to one or more hydraulic cylinders used to control the trim or tilt of a marine drive unit relative to a gimbal housing.
Unpublished U.S. patent application Ser. No. 14/873,803, filed Oct. 2, 2015, and assigned to the Applicant of the present application, discloses systems and methods for controlling position of a trimmable drive unit with respect to a marine vessel. A controller determines a target trim position as a function of vessel or engine speed. An actual trim position is measured and compared to the target trim position. The controller sends a control signal to a trim actuator to trim the drive unit toward the target trim position if the actual trim position is not equal to the target trim position and if at least one of the following is true: a defined dwell time has elapsed since a previous control signal was sent to the trim actuator to trim the drive unit; a given number of previous control signals has not been exceeded in an attempt to achieve the target trim position; and a difference between the target trim position and the actual trim position is outside of a given deadband. The method may include sending a second control signal for a defined brake time to trim the drive unit in an opposite, second direction in response to a determination that the actual trim position has one of achieved and exceeded the target trim position.
Each of the above U.S. patents and applications is hereby incorporated herein by reference.